Electrically conductive powders are used as fillers to form electrically conductive layers for recording papers or to prepare electrically conductive plastics or antistatic plastics. Electrically conductive powders may be incorporated in magnetic tapes or recording sheets to reduce their electrostaticity.
As such electrically conductive powders, carbon black powders are now most generally used, since they are inexpensive and have electrical conductivity suitable for the abovementioned uses, that is, a specific resistance on the order of 1 to 10.sup.-2 .OMEGA.cm. However, it is not easy to disperse carbon black powders uniformly in some resins, because the surface thereof is hydrophobic. In addition, some carbon black powders may contain carcinogenic 3,4-benzpyrene, which is derived from the raw material from which the carbon black powders are prepared. Therefore, there is an increasing concern over the safety of the carbon black powder for users and workers who handle it.
Carbon black powders are also used as black pigments or colored fillers. But because of their possible toxicity, it is not proper to use them in the cosmetic or food industry. Therefore, there is a demand for non-toxic blue or black pigments in these industrial fields, although electrical conductivity is not necessarily required in the materials used in these fields. Needless to say, pigments used for human skin or colored fillers used for food container plastics must be harmless.
When carbon black powders are used as black pigments, they are usually mixed with other pigments such as titanium dioxide (TiO.sub.2), and are dispersed in a fluid material. But it is sometimes difficult to maintain a stable uniform dispersion due to the fact that the specific surface area of the carbon black powder is much greater than that of the other pigments. Also the above-mentioned toxicity of carbon black poses serious problems against the usage in cosmetic industry.
Stannic oxide (SnO.sub.2) powder, copper iodide (CuI) powder, etc. are also known as electrically conductive powders. However, it is difficult to steadily produce these powders with uniform quality. In addition, these materials are toxic, too.
Magnetite (Fe.sub.3 O.sub.4) is also known as a black pigment, but its thermal instability and aggregativeness due to the magnetism possessed by the powder limit its usage.
On the other hand, the so-called "lower titanium oxide powder", a non-stoichiometric compound, the O/Ti gram-atom ratio of which is less than 2 can be easily dispersed in nearly all resins, because it has an affinity for resins stronger than the other electrically conductive powders such as carbon black powders. Another advantage of the lower titanium oxide is the fact that the specific resistance thereof can be varied over a wide range of 10.sup.3 to 10.sup.-2 .OMEGA.cm depending on the degree of reduction when it is made from TiO.sub.2. Therefore, for a particular purpose a powder with a desired electrical conductivity can be selected. The O/Ti gram-atom ratio of stoichiometric titanium dioxide is 2, while the lower titanium oxide has an O/Ti gram-atom ratio less than 2 and the ratio varies with the degree of reduction. For example, Ti.sub.6 O.sub.11, Ti.sub.7 O.sub.13, Ti.sub.9 O.sub.17 etc. are known. Generally, the specific resistance of the lower titanium oxide powder decreases as the O/Ti gram-atom ratio decreases.
The color of lower titanium oxides varies depending on the degree of reduction as well, and so it exhibit various tints such as greenish gray, bluish gray, bluish black, black, purplish black, bronze, etc. In addition, the lower titanium oxide is non-toxic so thaft the safety of users and workers is ensured. Therefore, this material has been expected to be used for many purposes such as afore-mentioned electrically conductive filler and gray or black pigment. However, it was difficult as described hereinafter to obtain a lower titanium oxide powder which is very fine and highly uniform in particle size, although such a powder is required as an electrically conductive material and as a pigment. Therefore the lower titanium oxide powder has not been practically used heretofore.
A process for preparing the lower titanium oxide was reported by P. Ehrlich (Z. Elektrochem., 45, 362 (1939)). The process comprises heating a mixture of titanium dioxide (TiO.sub.2) powder and metallic titanium powder is vacuo. According to said process, oxidation-reduction proceeds between the solid phases of titanium dioxide and metallic titanium. Therefore a lower titanium oxide with a desired electrical conductivity or of a desired tint can be produced by changing the mixing ratio of the starting titanium dioxide and metallic titanium. However, since said solid phase reaction is usually carried out at a high temperatures of 1000.degree. to 1600.degree. C., the particles of the mixed powder are inevitably sintered, and therefore the particle size increases during the reaction and the particle size of the resulting lower titanium oxide is liable to be non-uniform. In addition, said process requires a relatively long time to complete the reaction. In order to obtain a lower titanium oxide powder having a desired particle size by said process, the particle size of the starting material must be considerably smaller than the particle size desired in the product powder, because particles sinter and grow larger during the reaction. It is desirable that the particle size of an electrically conductive material or a pigment be as small as possible, preferably not greater than 1.0 .mu.m and that said particles be highly uniform in size. However, since the metallic titanium powder having a particle size of 1.0 .mu.m or less is not readily available at present. Even if a metallic titanium powder having an average particle size of about 1 .mu.m were used as the raw material, some amount of excessively large particles, which are not appropriate for practical use, would be inevitably formed. Further, the uniformity of the thus obtained powder would be low because of the irregular particle growth. As described above, said prior art process can not produce a lower titanium oxide powder having the desired properties.
Another process for preparing the lower titanium oxide was reported by P. Ehrlich (Z. Anorg. Allg. Chem., 247, 53 (1941)). The process comprises heating titanium dioxide powder in a hydrogen atmosphere to reduce the O/Ti gram-atom ratio of the powder. In this process, a temperature of 950.degree. C. or higher must be employed. However, under such a high temperature, particles significantly sinter and grow larger so that excessively large particles are produced.
This invention is based on the discovery of the fact that when titanium dioxide powder is reduced in an atmosphere of ammonia gas instead of hydrogen gas, the reduction of TiO.sub.2 can proceed at a low temperature even under 950.degree. C. and can be completed within a short time. Accordingly the processing temperature for reduction of TiO.sub.2 can be lowered to under 950.degree. C., and therefore the particle size of the product powder remains as fine as that of the starting TiO.sub.2 powder.